Basin Effects on the Seismic Fragility of Steel Moment Resisting Frames Structures: Impedance Ratio, Depth, and Width of Basin

Abstract

Deep sedimentary basins often increase the intensity of ground motions, but this effect is not considered explicitly in most codal provisions. The effect of basin amplification on structures to the fragility level is significant to study. For the first time, the effect of basin amplification on Steel Moment Resisting Frames (SMRF) is presented as a function of the basin material and geometry. This paper evaluates the effect of basin material, basin depth, and basin width on Peak Ground Velocity (PGV), Spectral Acceleration (Sa), and fragility of 4, 8, 12, and 20 story steel structures using synthetic ground motions simulated in SPECFEM3D. It has been found that the variation in basin width and impedance ratio can increase the spectral acceleration by a factor of 4 and 2.5, respectively. The response of SMRF is computed by incremental dynamic analysis, and fragility curves are derived for the collapse limit state. Results of fragility analysis reveal that SMRF structure is more fragile to variation in impedance contrast between basin-bedrock. It has been observed from the results that the collapse intensity measure for impedance ratio variation is 40% and 19% lesser on average than the width and depth variation, respectively. Comparison between the present fragility analysis results and HAZUS fragility parameters indicates that the vulnerability of structure located in the basin is underestimated in its current provisions, and the SMRF would need to increase its strength two times to account for basin amplification.